merge branch 'pr-711'
merge branch 'pr-711'

  conversion: add platform conversions

LGTMs: eea14ee25bc91923c994adfab6a0b5b2cecbef8e@stevvooe
Closes #711

opencontainers/image-spec,jonboulle/image-spec,jonboulle/image-spec,opencontainers/image-spec